The effects of soil and water contamination by lead (Pb) and the consequences on plant growth and yield are of great concern worldwide. Limits of the Pb concentration in water have been established by governmental institutions but these differ from each other. In this study, Lactuca sativa (var. Reine de Mai) plants were exposed to low Pb(NO3)(2) doses (0.05-20 mg L-1), including the recommended limit values for irrigation water by the Food and Agriculture Organization (FAO). After 28 d of exposure, lettuce plants did not present visible morphological alterations or growth impairment, but CO2 assimilation rate (P (N)), photochemical quenching, and effective quantum efficiency of PSII were negatively affected, while intercellular CO2 concentration, stomatal conductance, or transpiration rate were not influenced. Our results suggested that limitations on photosynthesis occurred from different reasons than due to the decrease of internal CO2 availability, alterations of photophosphorylation, and/or electron transport rate. Thus, this lettuce cultivar showed photosynthetic susceptibility to low doses of Pb, even at lower concentrations than those maximal allowed for irrigation water by FAO. Furthermore, P (N) seemed to be the most sensitive biomarker for evaluation of Pb susceptibility.

FCT/MCT supported S. Silva (FCT/BPD/74299/2010) and G. Pinto (SFRH/BPD/101669/2014) grants from the financing program QREN-POPH/FSE - Tipologia 4.1 - Formacao Avancada. We thank for the financial support to CESAM (UID/AMB/50017), to FCT/MEC through national funds, and the co-funding by the FEDER, within the PT2020 Partnership Agreement and Compete 2020, also this work was funded by FEDER/COMPET/POCI, POCI-01-0145-FEDER-006958 (UID/AGR/04033/2013) and UI QOPNA (Ref. FCT UID/QUI/00062/2013).